Figure 26 shows the simplified version of the feedback network. The accuracy of the output voltage is dependent on the regulation voltage accuracy of the TL431LI. A simplified VOUT can be seen in Equation 2 but this equation does not include errors that will deviate the output.
The primary sources of error are the Error|Vref and Error|Iref. The Error|Vref primarily consists of the errors that affect the internal bandgap voltage reference of the TL431LI. This consists of errors from the initial accuracy, temperature drift, ratio of change in reference voltage to the change in cathode voltage, and dynamic impedance. The benefit of the TL431LI is its low temperature drift, VI(dev), which allows the Vref to be more accurate across the full temperature range compared to typical TL431LI devices. Equation 3 shows a simplified worst case Vref with initial accuracy and temperature drift.
The Error|Iref in Figure 26 is dependent on the Iref and II(dev) along with R1. The TL431LI has improved Iref and II(dev) which allows the values of the resistor R1 to be increased to save power. Typically optocoupler feedback design requires the Iref to be taken into account when doing VOUT calculations but the error comes from the deviation from the maximum to typical value of Iref. In addition to this, the II(dev) is the temperature deviation on the Iref current which will affect the overall reference current into the TL431LI. Equation 4 shows the VOUT of the TL431LI for Figure 26 which includes the improved Iref and II(dev). The VOUT equation assumes that the resistors R1 and R2 have a 0.5% accuracy tolerance.
Comparing the calculated VOUT without and without error the expected worst case max error is 2.1% which meets the 3% error target.